Serpent Surprise

The photographs on these pages involve a certain amount of intention to deceive, something this magazine normally does not do. The fearsome monster at left is actually a garter snake, the same small, harmless creature the little girl is playing with at right. Enlargement works wonders. The photograph that fills the following pages depicts a matter of some delicacy. That jumble is composed entirely of males waiting for a female to emerge from a den. They are used to crowding: members of this species form the largest concentration of snakes in the world and one of the largest concentrations of vertebrates anywhere. "A single den," says Oregon State snake biologist Robert Mason, "is about the size of an average American’s living room and may hold up to 25,000 snakes."

 Various species of garter snake are a familiar sight in many parts of North America. But the red-sided garter snake, native to central Manitoba, would appear to be in a class by itself. At the very northern limit of where reptiles can live, it hibernates for a full eight months. Because it has to find dens more than six feet down to escape the killing frost, suitable sites are hard to come by. In spring, 10,000 or more males may be found at the entrance to one den.

Those thousands of males are waiting for females. According to photographer Nic Bishop: "Each is immediately swarmed over by the sexually fervid males, which engulf her in a 'mating ball' of as many as 100 individuals. Even if she is smothered to death (the snakes can pile up to two feet high), she is no less eagerly courted, because the male’s mating instinct is elicited by a sex pheromone, an odor signal that animals use to communicate with each other, contained in the female’s skin. Only after all the females have left the dens, which takes about a month, do the males disperse to their summer feeding grounds."

 Red-sided garter snakes are remarkable for other qualities as well. They may travel as far as 20 miles from their dens, following pheromone trails. They can follow a trail left by just one snake, and the latest research indicates they can tell which way that snake was traveling. Despite being well studied, however, they retain some of their mystery. We don’t know whether or how the snakes distinguish pheromones that lead to their particular den. It may even be that they navigate by the sun. No one knows where the babies born in the summer spend their first winter. They start using dens only in the second winter. No one knows if and how any genetic mixing between dens takes place, and what role the babies may play in it.

Not everyone would wish to come upon the scene on the next pages. Those swarming snakes are no more a threat than a single one in a garden in Ohio, however. And they do remind us of nature’s seemingly boundless improvisation of behavior.

-John P. Wiley, Jr.

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